Historically, companding is a widely used technique for gaining a signal to noise ratio improvement in systems where a signal is passed through a noisy transmission medium. Companding is utilized when one wishes to transmit a signal with a large dynamic range through a channel that has limited dynamic range. The process of companding is a method whereby data with a large dynamic range is first "compressed" thereby attenuating the high voltage signals and amplifying the low voltage signals. This compressed limited dynamic range signal is then usually transmitted over a channel. Upon receipt, the data is "expanded" thereby amplifying the high voltage signals and attenuating the low voltage signals.
The basic building blocks of a compander are an operational amplifier (op amp), a full wave rectifier/averaging circuit and a variable gain stage. The op amp is typically connected in a negative feedback mode whereby the full wave rectifier/averaging circuit and the variable gain stage are either in the feedback loop for compressor implementation or connected to the inverting input of the op amp for expander implementation.
The full wave rectifier/averaging circuit performs full wave rectification of the input signal, for the expander, or of the output signal, for the compressor, and then averages the rectified signal to obtain a voltage signal that is proportional to the average level of the input or output signal for a expander or compressor, respectively. This signal then feeds the variable gain stage which determines the overall gain of the compander.
Using a compander for applications which require large dynamic range input signals has been difficult in the past. This is because most, if not all, prior art has performed the full wave rectification by an NPN/PNP transistor combination of Q1 and Q2, as shown in FIG. 1. Error arises due to the beta error that inherently exists between NPN transistor Q1 and PNP transistor Q2, especially for high currents. Q1 and Q2 will also track differently with respect to temperature and process. Also the negative going input signal passing through Q1 is mirrored by M2 before averaging, while the positive going input signal passing through Q2 is not mirrored before averaging. This produces additional beta errors due to the mirror M2.
Another reason that prior art is difficult to use for large dynamic range input signals is due to the varying resistance of D1, shown in FIG. 1, for varying input signal levels. For large currents the resistance of diode D1 is typically much smaller than R.sub.A, thereby R.sub.A dominates the RC time constant as intended. However, for small currents the resistance of diode D1 is much larger than R.sub.A, thereby increasing the effective resistance from R.sub.A to R.sub.A +R.sub.D1. This will have an undesirable effect of increasing the RC time constant thereby increasing the time to reach a steady state voltage.
One attempt that prior art has made to maintain a constant resistance of D1 has been to pre-bias the diode D1 with a current source so that D1 is always on. However, an equivalent current source must exist at the output to cancel the pre-bias current in I.sub.AVE. The problem with this method surfaces when the pre-bias current must be cancelled to obtain an accurate representation for very small I.sub.AVE. This requires a current source to be accurate to nanoamperes.
In order to utilize a full wave rectifier/averaging circuit for large dynamic range input signals, the beta errors need to be minimized and the RC time constant needs to be fixed for small as well as large signals.
Hence, a need exists for an improved circuit and method for providing a full wave rectifier/averaging circuit in transmission systems.